Confined spaces such as building interiors and packaging may have limited exchange of moisture vapor with adjacent or external environmental spaces. Materials having moisture permeance that is variable when exposed to different ambient conditions can provide benefits in such fields as construction, packaging, etc.
Buildings are commonly equipped with thermal insulation in order to provide comfortable living conditions inside the building. In addition to insulation, buildings are also constructed with various air barrier materials to limit air and water infiltration into the structure of the building. These air barriers minimize heat loss through draft flow.
In addition to air barriers, vapor barriers have also been used for controlling flow of moisture. Such vapor barriers reduce the potential for moisture condensation in the interior wall cavity space as the water vapor is exposed to the cold exterior temperatures in the winter season. A material that acts as a vapor barrier during winter but facilitates vapor permeation during hot, humid summer conditions is desirable. U.S. Pat. Nos. 6,808,772; 6,878,455 and 6,890,666 disclose applications of a polyamide building liner material that has a water vapor diffusion resistance (WVDR) of from 2 to 5 meters diffusion-equivalent air space width at a relative humidity (RH) of an atmosphere surrounding the vapor barrier between 30% and 50% and a WVDR of less than 1 meter diffusion-equivalent air space width at a relative humidity between 60% and 80%. See also, Patent Application Publications US2003/0215609.
Variable vapor permeance can also be desirable of packaging for packaging of food items such as fresh produce to have low moisture permeance under conditions of typical cold storage (low temperatures and low humidity) to retain moisture within the package, thereby retarding desiccation of the produce and maintaining freshness.
However, water vapor transmission rate (WVTR) of variable vapor barrier increases in a logarithmic manner as relative humidity increases. It is desirable to limit the maximum WVTR, or to specify the relative humidity at which the WVTR increases dramatically.